This invention relates to wireless data communications systems and particularly to wireless data communications systems which include arrangements for locating mobile units within the area serviced by the system.
The use of mobile data communications systems to perform location functions for locating mobile units is described in articles entitled Enhancements to the User Location and Tracking System, by Paramvir Bahl and Venkata N. Padmanabhan, and User Location and Tracking in an In-Building Radio Network, Microsoft Technical Report MSR-TR-99-12, dated February 1999, by the same authors, both published by Microsoft Research. As described therein signal strength of signals of the wireless data communications system, such as a system using the protocol of IEEE Standard 802.11, are used for locating mobile units within an area serviced by the system. Other techniques for locating mobile units using the wireless data communications systems or other location system are possible.
In co-pending application Ser. No. 09/528,697, filed Mar. 17, 2000, which is owned by the assignee of the present application and incorporated herein by reference, there is described a system which follows the protocol of IEEE Standard 802.11, but which uses a combination of RF Ports and Cell Controllers to perform the functions of Access Points of a classical 802.11 data communications system. Lower level MAC functions are performed by the RF Ports and higher level MAC functions, including association and roaming functions, are performed by the cell controller. The term “access point” as used herein is intended to include conventional access points, such as those which follow the protocol of IEEE Standard 802.11 and perform all MAC functions, as well as RF Ports operating with cell controllers, as described in the incorporated co-pending application.
One problem associated with locating mobile units using a wireless local area network system is the comunication burden the location function can put on the system. In a passive system wherein mobile units periodically listen for beacon signals from access points or other beacon transmitters for location determination, the mobile units must monitor beacon signals on a plurality of channels, such as 11 channels for IEEE Standard 802.11 (b) systems. Considerable operating time for a mobile unit can therefore be spent for passive monitoring of 11 channels for signals, such as beacon signals, to be used for location determination. Further, such passive monitoring can reduce the amount of time a mobile unit can operate in power save mode, therefore shortening battery life. The larger number of beacon signals that are required for the location function can also increase radio interference with data communications functions. Likewise, in arrangements wherein location signals are transmitted by reference signal transmitters or access points in response to request signals transmitted by mobile units, the request signal adds to the radio background and can further burden the media and the battery requirements of mobile units.
It is an object of the present invention to provide an improved method and system for locating mobile units using a wireless data communications system.